git-arc: use `${ARC_CMD}` instead of `arc`

[ngie]

Summary

This change helps further ensure that archivers/arc can be installed
along with devel/freebsd-git-tools.

MFC after: 1 week

[dteske]

Not necessary. See line 168 (arc())

tools/tools/git/git-arc.sh

168

This line right here means that when we say arc in this script, we are not using PATH expansion we are using the function which calls ${ARC_CMD}

[ngie]

tools/tools/git/git-arc.sh

168

This line right here means that when we say arc in this script, we are not using PATH expansion we are using the function which calls ${ARC_CMD}

Ah, derp. Yes, mea culpa.